This invention relates to a method and apparatus for the continuous refining of iron or iron alloys by continuously conducting desulfurization or dephosphorization of the molten pig iron discharged from a blast furnace or by the continuous manufacture of the case iron by adding ferrosilicon.
Demand for steel of low sulfur content, for example of less than 0.005% or less than 0.015% has recently been increased. On the other hand, a decrease in the coke ratio is strongly desired from the standpoint of the shortage of the supply of coking coal. For this reason, heavy oil or other liquid fuel is used to save coke consumption in the blast furnace. But due to shortage of heavy oil of low sulfur content it is inevitable that pig iron containing a large quantity of sulfur is produced. Consequently, it is highly desirable to provide an efficient method and apparatus capable of readily removing sulfur from a large quantity of molten pig iron or steel.
Phosphorus has also been considered one of the harmful elements as sulphur. However, phosphorus can be relatively readily removed by the basic steel making operation and a number of methods for this purpose have been proposed. Thus phosphorus did not present any serious problem. However, demand for steel of low phosphorus content has also increased because it was found that in high tension steels, especially having a tensile strength of 80 kg/mm.sup.2, decrease in the phosphorus content is an efficient means for preventing the formation of weld cracks. As above described, it has been considered that the dephosphorization of steel is relatively easy but due to the increase of the size of LD converters, the conventional method of dual slag removal has become difficult to practice. Further, the method of increasing basicity beyond a normal value is not always an effective process because this method increases the losses of iron and heat.
While the above description refers to a case wherein molten pig iron is used as the raw material for manufacturing steel, the method of preparing cast iron for use of casting from pig iron originally intended to manufacture steel has recently become noteworthy. The pig irons for casting and steel making are different in the content of silicon. More particularly, the content of silicon in the steel making is about 0.7%, whereas in the latter it is more than 2%. For this reason, it is possible, theoretically, to convert the pig iron for steel making into the cast iron by addition of ferrosilicon. However, this method has not been practiced widely since it is necessary to consider other compositions and various other factors. Accordingly, it has been the practice to manufacture the cast iron by using a relatively small blast furnace intended for its purposes only, or to prepare the two different types of pig iron alternatively in one blast furnace.
However, recent tendency of increasing the capacity of blast furnaces makes it impractical to alternately prepare the pig iron for steel making and the cast iron for casting. Accordingly, it is necessary to use small blast furnaces for preparing the cast iron, but the use of such small blast furnaces cannot reduce the manufacturing cost as in the case of using large size blast furnaces.
Of course, it may be possible to prepare the cast iron with a large blast furnace. However, recent increase in the production capacity of a single blast furnace is much larger than the increase in the demand of cast iron. Therefore, if it were possible to efficiently convert the pig iron for steel making into the cast iron, it would be possible to meet the gradual increase in the demand of cast iron and to fully enjoy the advantage of the large capacity blast furnace. From the standpoint of engineering, it is possible to add ferrosilicon at a high efficiency yield to the molten pig iron, but the resulting cast iron contains phosphor of below a certain limit because it is impossible to sufficiently dephosphorize.
In addition, the resulting cast iron does not contain sufficient quantities of various valuable compositions, such as Cr, Ti, V, S, etc. which are essential to ductile cast iron, for example, the demand thereof increasing rapidly in recent years.
To this end, although a number of methods of desulphurization have been proposed in the past, so long as the applicant is aware, no proposal has actually been practiced. Most noteworthy reasons common to these prior methods are as follows. Because a large quantity of pig iron is treated at a time, the useful life of the container or the like is relatively short, thereby requiring frequent repairs and because the close contact between a desulfurization agent and the molten pig iron is not possible with the large amount of desulfurization that is required. Thus, it is necessary to use a large quantity of the desulfurization agent. Moreover, a shallow depth of the molten pig iron in a tank for heating the same and a short contact interval between the desulphurization agent and the molten pig iron cannot assure sufficient desulphurization. Accordingly, it is necessary to provide an extremely wide surface area of the molten pig iron in order to attain a high percentage desulfurization. For example, in a method of using an electromagnetic pump for stirring the molten pig iron by magnetic field, it was found that the percentage of desulfurization was only about 50% throughout the entire length of vessel, having a total length of 6 m. FIG. 1 is a schematic representation of the apparatus to work out the method proposed by Rhein Stahl Aktiengesselshaft of West Germany which is said to be the most efficient method available at present.
This method shows a fairly high percentage of desulfurization in spite of shallower depth of the molten pig iron when compared with other methods. More particularly, as shown in FIG. 2, (a) when 5 to 7 kg calcium carbide per ton of molten pig iron is used as the desulphurization agent, the percentage of desulphurization amounts to 70%, and (b) when 9 to 10 kg and (c) 13 to 14 kg of the desulphurization agent per ton of molten pig iron are used, the percentage of desulphurization amounts to 82% and to about 90%, respectively.
According to the manufacturer's announcement, the apparatus shown in FIG. 1 has an inner diameter of two meters, the depth of the molten pig iron of about 30 cm, flow quantity of the pig iron of 6 tons/min and an average residence time of the pig iron in the treating tank of a little less than one minute. The reason that the apparatus can realize a relatively good result notwithstanding such a short residence time is considered as caused by a good stirring action.